Subject: Re: [PATCH] powerpc/process, kasan: Silence KASAN warnings in __get_wchan()

From: Kees Cook <[email-masked]>
Date: Wed, 19 Jan 2022 10:51:34 -0800

On Tue, Jan 18, 2022 at 08:50:25PM -0500, He Ying wrote:
> The following KASAN warning was reported in our kernel.
> 
>   BUG: KASAN: stack-out-of-bounds in get_wchan+0x188/0x250
>   Read of size 4 at addr d216f958 by task ps/14437
> 
>   CPU: 3 PID: 14437 Comm: ps Tainted: G           O      5.10.0 #1
>   Call Trace:
>   [daa63858] [c0654348] dump_stack+0x9c/0xe4 (unreliable)
>   [daa63888] [c035cf0c] print_address_description.constprop.3+0x8c/0x570
>   [daa63908] [c035d6bc] kasan_report+0x1ac/0x218
>   [daa63948] [c00496e8] get_wchan+0x188/0x250
>   [daa63978] [c0461ec8] do_task_stat+0xce8/0xe60
>   [daa63b98] [c0455ac8] proc_single_show+0x98/0x170
>   [daa63bc8] [c03cab8c] seq_read_iter+0x1ec/0x900
>   [daa63c38] [c03cb47c] seq_read+0x1dc/0x290
>   [daa63d68] [c037fc94] vfs_read+0x164/0x510
>   [daa63ea8] [c03808e4] ksys_read+0x144/0x1d0
>   [daa63f38] [c005b1dc] ret_from_syscall+0x0/0x38
>   --- interrupt: c00 at 0x8fa8f4
>       LR = 0x8fa8cc
> 
>   The buggy address belongs to the page:
>   page:98ebcdd2 refcount:0 mapcount:0 mapping:00000000 index:0x2 pfn:0x1216f
>   flags: 0x0()
>   raw: 00000000 00000000 01010122 00000000 00000002 00000000 ffffffff 00000000
>   raw: 00000000
>   page dumped because: kasan: bad access detected
> 
>   Memory state around the buggy address:
>    d216f800: 00 00 00 00 00 f1 f1 f1 f1 00 00 00 00 00 00 00
>    d216f880: f2 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>   >d216f900: 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00
>                                             ^
>    d216f980: f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 00 00
>    d216fa00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> 
> After looking into this issue, I find the buggy address belongs
> to the task stack region. It seems KASAN has something wrong.
> I look into the code of __get_wchan in x86 architecture and
> find the same issue has been resolved by the commit
> f7d27c35ddff ("x86/mm, kasan: Silence KASAN warnings in get_wchan()").
> The solution could be applied to powerpc architecture too.
> 
> As Andrey Ryabinin said, get_wchan() is racy by design, it may
> access volatile stack of running task, thus it may access
> redzone in a stack frame and cause KASAN to warn about this.
> 
> Use READ_ONCE_NOCHECK() to silence these warnings.
> 
> Signed-off-by: He Ying <[email-masked]>

Looks reasonable to me; thanks!

Reviewed-by: Kees Cook <[email-masked]>

-Kees

> ---
>  arch/powerpc/kernel/process.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 984813a4d5dc..a75d20f23dac 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -2160,12 +2160,12 @@ static unsigned long ___get_wchan(struct task_struct *p)
>  		return 0;
>  
>  	do {
> -		sp = *(unsigned long *)sp;
> +		sp = READ_ONCE_NOCHECK(*(unsigned long *)sp);
>  		if (!validate_sp(sp, p, STACK_FRAME_OVERHEAD) ||
>  		    task_is_running(p))
>  			return 0;
>  		if (count > 0) {
> -			ip = ((unsigned long *)sp)[STACK_FRAME_LR_SAVE];
> +			ip = READ_ONCE_NOCHECK(((unsigned long *)sp)[STACK_FRAME_LR_SAVE]);
>  			if (!in_sched_functions(ip))
>  				return ip;
>  		}
> -- 
> 2.17.1
> 

-- 
Kees Cook